The trail over Col du Brévent and Le Brévent should not be attempted in bad weather, if a storm is forecasted or there is a chance of ice and or snow on the ground. Check the weather conditions locally with the Refuge de Moëde d'Anterne before deciding which route to take. Descend on foot to the hamlet of Servoz from where you can catch a train to Chamonix: https://www.sncf.com/fr